nuke: add missing import os

$ teuthology-nuke  -a . -r -u
Traceback (most recent call last):
  File "/home/ubuntu/bin/teuthology-nuke", line 9, in <module>
    load_entry_point('teuthology==0.0.1', 'console_scripts', 'teuthology-nuke')()
  File "/home/ubuntu/teuthology/teuthology/nuke.py", line 343, in main
    ifn = os.path.join(ctx.archive, 'info.yaml')
UnboundLocalError: local variable 'os' referenced before assignment

Signed-off-by: Sage Weil <sage@inktank.com>
If we're scheduling for non-ubuntu, look for rpms
Add missing requirement: web.py
add flake8 checks to teuthology

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
(cherry picked from commit 27c9eb8f81dc25c7b50bfb77d4d2d4ab4d136fb6)
Make teuthology.locker a module
Fix undefined name error
Remove unused imports
Remove unused variables
Don't redefine variable